1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus incorporating the same, and more particularly, to a fixing device that fixes a toner image in place on a recording medium with heat and pressure, and an electrophotographic image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of those imaging functions, incorporating such a fixing device.
2. Description of the Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium by melting and settling the toner with heat and pressure.
Various types of fixing devices are known in the art, most of which employ a pair of generally cylindrical looped belts or rollers, one being heated for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together forma heated area of contact called a fixing nip through which a recording medium is passed to fix a toner image onto the medium under heat and pressure.
One such fixing device includes a multi-roller, belt-based fuser assembly that employs an endless, flexible fuser belt entrained around multiple rollers, one of which is equipped with an internal heater to heat the length of the fuser belt through contact with the heated roller. The fuser belt is paired with a pressure roller pressed against the outer surface of the fuser belt to form a fixing nip therebetween, at which a toner image is fixed in place with heat from the fuser belt and pressure from the pressure roller.
Another type of fixing device includes a film-based fuser assembly that employs a fuser belt formed of thin heat-resistant film cylindrically looped around a stationary, ceramic heater. The fuser belt is paired with a pressure roller that rotates while pressing against the stationary heater through the fuser belt to form a fixing nip therebetween. At the fixing nip, the pressure roller rotates to advance the fuser belt together with an incoming recording sheet, while the stationary heater heats the recording sheet via the fuser belt, so that a toner image is fixed in place with heat from the stationary heater and pressure from the pressure roller.
Owing to the heat-resistant film which exhibits a relatively low heat capacity and therefore can be swiftly heated, the film-based fuser assembly eliminates the need for keeping the heater in a sufficiently heated state when idle, resulting in shorter start-up time and smaller amounts of energy wasted during standby, as well as a relatively compact size of the fuser assembly. With its high processing speed and good thermal efficiency, this type of fixing device finds application in on-demand, energy-efficient printers.
One important factor that determines imaging quality of a fixing device is the ability to properly convey a recording medium through the fixing nip without wrapping around the rotary fixing member, which would otherwise result in concomitant jam or other conveyance failure. For obtaining a fixing process with high immunity against media conveyance failure, most fixing devices employ a media stripping mechanism that allows a recording medium to properly separate from the rotary member upon exiting the fixing nip. Typical of such media stripper is an elongated plate having multiple fingers arranged in a longitudinal direction along the axial length of a rotary fixing member, with their finger tips touching the fixing member to strip off a recording medium adhering to the circumferential surface.
A problem encountered when employing a multi-fingered media stripper with a rotary fixing member, in particular, a rotary pressure member or roller, is variations in surface roughness of the fixing member caused during operation. That is, holding the stripper in direct contact with the rotating pressure roller causes scratches or gouges on those portions of the pressure roller where the finger tips rest on the moving surface of the pressure roller, while leaving the other portions substantially intact and smooth.
Such variations in surface roughness of the pressure member adversely affect quality of duplex printing through the fixing nip, where a recording medium, having its first side previously printed, undergoes a second pass to fix a toner image in place on its second side. As the recording medium enters the fixing nip, heat from the pressure member causes toner once set on the first printed side to slightly melt and soften to conform to the circumference of the pressure member, which makes the toner image appear rough and uneven in texture and gloss, as if copied from the roughened surface of the pressure member.
The problem is particularly pronounced where image formation is performed using toner that melts at a melting temperature significantly lower than those exhibited by standard types of toner. Toner with low melting point is developed to meet ever-growing demands for high-speed, energy-efficient printers, as it effectively reduces time and energy required to heat a fixing member to an operating temperature sufficient for fusing toner, resulting in short warm-up time for warming up the fixing assembly and short first-print time for receiving, executing, and completing an initial print job upon start-up.
To counteract the problem depicted above, one possible approach is to use a media stripper that can strip recording media without touching the pressure member. Unfortunately, however, this approach is impractical since using such a media stripper limits flexibility in optimizing certain operational properties of the fixing device, such as processing temperature, type of recording media, etc., which makes it difficult to provide a fixing process with high immunity against media conveyance failure.